1. Field of the Invention
The present invention relates to a negative-sequence time-overcurrent relay which is adapted for use in a polyphase synchronous rotary electric machine, such as a polyphase synchronous motor, polyphase synchronous generator and polyphase synchronous condenser, and protects the polyphase synchronous rotary electric machine.
2. Description of the Related Art
With regard to a synchronous rotary electric machine, the withstanding of negative sequence current is defined as follows. When a negative sequence current flows across the armature winding, a rotating field is generated on the rotator, causing an induced current to flow on the surface of the rotator. The surface temperature of the rotator therefore rises. To restrict the rising of the surface temperature of the rotator, the negative sequence current should be restricted.
There are two ways of restricting the negative sequence current: short withstanding for restricting a relatively large negative sequence current in a short-time area where the effect of the heat discharged from the rotator is negligible and continuous withstanding for restricting a relatively small negative sequence current for a long period of time or in a continuous-time area where the effect of the heat discharged from the rotator appears.
When a harmonic current flows across the armature winding of a synchronous rotary electric machine, a rotating field having a frequency several times greater than that of the harmonic current is generated in the rotator. Consequently, an induced current flows on the surface of the rotator, causing the surface temperature of the rotator to rise. Like the restriction of the negative sequence current, the harmonic current is also restricted.
Therefore, the characteristics of a synchronous rotary electric machine are restricted by a total negative sequence current (I.sub.2), which is the total influence of the negative sequence current of the fundamental wave and the harmonic current.
When the fundamental-wave negative sequence current (I.sub.2.multidot.1f) and an arbitrary harmonic current (I.sub.nf : n being the order number of the harmonic) flow in a synchronous rotary electric machine, the amount of their influence, if expressed as the total negative sequence current I.sub.2, is defined by the following equation (1). Here the total negative sequence current I.sub.2 is the amount of the influence of the fundamental-wave negative sequence current and an arbitrary harmonic current on the synchronous rotary electric machine, expressed by the fundamental-wave negative sequence current which imparts the same amount of influence on the synchronous rotary electric machine. ##EQU1## where n is the order number of the harmonic, I.sub.1.multidot.nf is the positive sequence current of the n-th harmonic, and I.sub.2.multidot.nf is the negative sequence current of the n-th harmonic.
In the definition equation (1), the zero-phase component of the harmonic current is harmonic current is omitted because it hardly flows due to the center tap of the synchronous rotary electric machine being connected to a high resistor.
The following is one example of the total negative sequence current I.sub.2 when the fundamental-wave negative sequence current and the fifth and seventh harmonics are flowing. ##EQU2##
In computing the total negative sequence current when the fundamental-wave negative sequence current and harmonic current flow in the synchronous rotary electric machine as expressed in the definition equation (1), after the harmonic current is detected, the harmonic current is separated into a positive-sequence component and a negative-sequence component, which are then computed to be equivalent negative sequences.
Therefore, high-precision phase control and level control are needed, and the control circuit has many components, complicating the structure, and requires many control elements.
It is therefore an object of the present invention to provide a negative-sequence time-overcurrent relay which protects a synchronous rotary electric machine without separating a harmonic current negative sequence into a positive sequence and a negative sequence, requiring special phase and level controls and many control elements, and reducing the difference between the harmonic current and a total negative sequence current.